LONDON — Prime Minister Gordon Brown apologized Wednesday for a potentially disastrous government blunder in which two computer disks containing personal and financial data on 25 million Britons disappeared after a junior government minister said he handed them to a courier service.

“I profoundly regret and apologize for the inconvenience and worries that have been caused to millions of families,” Brown told the House of Commons, where the revelation created new embarrassment for a prime minister already feeling pressure from recent political missteps.

A government employee has told investigators that he downloaded the personal information, including bank account numbers, addresses and birthdates, put them on two disks and handed them Oct. 18 to the courier service TNT for delivery to another government office.

Though the data were password-protected, stronger security measures normally taken for such sensitive information were not applied — the information was not encrypted, and the package was not sent by TNT’s registered service. The disks have not been seen since.

TNT spokesman David Walker said his company was cooperating with police concerning the package, but “we can’t say for sure that it was ever in our possession.”

The fear is that the data could be used to steal people’s identities and commit fraud on a grand scale. Brown said there was no evidence that the information had fallen into criminal hands.

The loss of confidential information, believed to be the biggest ever in Europe, was made public Tuesday. Members of Parliament gasped in disbelief when Alistair Darling, chancellor of the exchequer, or finance minister, revealed the loss.

In a heated exchange in the House of Commons on Wednesday, David Cameron, leader of the opposition Conservative Party, told Brown that he should take more responsibility for this “appalling blunder.”

Simon Davies, a data security expert and visiting fellow at the London School of Economics and Political Science, said that without secure encryption, “there is very simple technology to break through password protection. It means very little.”

“At every level, security has been breached,” he said. “We all want to know who authorized the system that would allow the dumping of all of this data on two disks.”

The disks contain information on 7.2 million families — all those with a child and therefore eligible for child benefits — and 25 million individuals. That means it affects more than 40 percent of the British population of 60 million.

Davies said that a criminal could use as few as three items of data to impersonate someone, open accounts in the victim’s name and make purchases. The disks contain seven for each individual, making it a bonanza for commission of fraud.

The incident comes as Brown, who took office in June, is attempting to recover from the political beating he suffered in September, when he appeared to waffle over whether to call a snap general election. Brown ultimately decided against it, which Cameron and other opponents used to portray him as weak and politically clumsy.

The story dominated the radio and television airwaves in Britain on Wednesday as pundits and others expressed mocking disbelief. “He is in a bit of mess now,” said Patrick Dunleavy, a professor of politics at the London School of Economics.

He said that the British civil service, which has long regarded itself as the Rolls-Royce model of administration, also comes off poorly. The idea of a junior employee being able to download a large batch of confidential details off a database and give them to a courier “without internal controls, checks and balances … looks very bad,” he said.

Paul Gray, head of Revenue and Customs, the office in charge of the disks, resigned Tuesday.

Darling said that before the loss was made public, police had been notified and banks alerted to watch for any unusual activity in the affected bank accounts.

Banks have reported customers calling in to change their passwords or check on their accounts.

Tony Hayes, 79, a retired musician, went to his bank in north London on Wednesday to assure himself that his account was all right. “More care should have been taken” with the government data, he said.

More in News

As social media companies wrestle with how to police dangerous health misinformation on their platforms, Pinterest has taken an extreme approach: blocking search results related to vaccinations, whether the results are medically accurate or not.